The need for purification of water can arise in different connections. The purpose can be to provide pure drinking water for consumption, to prevent spill of water soluble or water transported contaminants, to get legal effluents from an industrial process, or to obtain desired components contained in the water, but in concentrations which are too low for general recovery. Primary there are two main groups of water purification; either for production of pure water or for recovery of substances which can be dissolved in the water. Different methods exist for purification of water; filtration, distillation, centrifugation, etc. Many methods are excellent for certain contaminants, but are ineffective for others. Only a few methods are good for all types of water soluble contents.
In connection with recovery of oil, it is a well known problem that also a water phase is produced. This water phase will be contaminated with, among others, hydrocarbons and salts. If this water, also called the “formation water”, “produced water” or “production water”, is dumped into the environment, in the sea or onshore, it will lead to a big contamination problem. Consequently it will be of importance to purify the water before it is dumped. One alternative is reinjection of the water into the reservoir, but this will entail a cost limiting factor for the oil recovery by the vary fact that there is a limitary factor as to how much oil which can be recovered from a reservoir if all of the produced water is reinjected.
In Norway it has been decided that after 2005 no effluent in the form of production water will be admitted. This means that the only effluent from the process will be pure water. Natural effluents from sediments will occur, but this fact is not taken into consideration here. The Norwegian requirement as to effluent has brought into focus the continuous increasing production of water from Norwegian oil wells.
There is a difference between a requirement of zero effluent in the sense of 0 ppm of anything, and zero harmful effluent. The latter means that a certain amount of substances up to a limit value of concentration and/or total amount and/or together with certain other compounds etc. might be acceptable. There exists many parameters in connection with harmfulness and many of these are hardly known. A further complicating element is that even if some substances are not harmful to the nature, even in big amounts, distinguishing between harmful and not harmful substances can be so problematic that it might happen that zero emission shows the best cost efficiency.
Troll B and C produce approximately 60 000 m3 formation water every 24 hours. This volume requires absolute purification to give an acceptable oil production. It means that a purification process has to handle 2 500 tons of water per hour. Solutions are installed for the purification of the water down to approximately 40 ppm hydrocarbon effluent (among others hydrocyclones, flocculation and filtering processes). In addition, unwanted salts might be discharged and the contaminating hydrocarbons are most likely particularly unwanted in the nature.
It might be possible to inject all of the produced water back into the reservoir from which the oil is recovered or another porous network over or below the producing zone. If all of the water is reinjected, it will, however, quite early in the production process become clearly unprofitable to produce, due to the high water content of the oil. This means that the degree of recovery gets lower than that which else could be reached. It is therefore an object of the present invention to provide a technological solution where absolute pure water is produced in big amounts from the formation water.
An important element of the present invention is that the contaminants are treated, either through recovery as a positive resource, or by safe deposit/disposal.
There exists alternative techniques for purification of water. A good description of these are given in Björn Hansen and Simon R H Davis, “Review of potential technologies for the removal of dissolved components from produced water”, IBC Offshore Water Management Conference, Aberdeen, 1993. Table 1 shows a summary of different techniques for purification of the produced water in connection with recovery of hydrocarbons.
TABLE 1Purification efficiency for different processes for purificationof produced water within the oil industryMethodPurification efficiency (removed amount)Ion exchange80% heavy metalsActivated carbon95% aromatics10% light hydrocarbons20—50% production chemicalsZeolite adsorption40—60% hydrocarbons,poor removal of added production chemicalsMembrane filter90% of BTX aromatics,15% of phenols,perhaps 10% of naphtenes“Air Stripping”ca. 90% of common hydrocarbons,does not remove phenols, fatty acids orproduction chemicalsBiologic treatmentGood on phenol and fatty acids (30—90%),for others bad
Since the present invention results in completely pure water, it is obvious that the method is much more efficient than the processes which are known from the earlier technique.
It has previously been shown in British patent GB 1.360.797, from 1973, that contaminated water can be purified through the formation of hydrates. This patent concerns the specific use of trichlorofluoromethane and 3-bromopropene separately or in mixture to make the hydrate. Nothing is mentioned about the fact that hydrates can be formed with hydrocarbon gases. It is a weakness with this patent that they use methane and propene halogenides. It is generally known that halogenides are not especially favourable when they are set free into the nature. Nor is the efficiency of the process with large amounts of dissolved salts, and how they will purify the water when the salt concentration gets high, described in the patent. It seems as if this is especially intended for contaminated surface water, e.g. from a river. In such water the target will be to take out pure water for consumption or industrial purposes, but there is no focus on what to do with the rest.
In U.S. Pat. No. 2,904,511, from 1955, it is also dealt with the production of pure water from salt water. This patent sets focus on the production of pure water from sea water, and it is based on taking out pure water by means of a hydrate process, whereas the more concentrated salt water which becomes the byproduct in the process, is discharged continuously. The elements with hydrate formation, hydrate harvesting, washing and also recirculation of hydrate forming compound and collection of pure water, are similar to our suggestions. This US-patent does not, however, take organic contaminants into consideration.
It is further shown in British patent GB 1.320.134, from 1973, as also in the two above mentioned (GB 1.360.797 and U.S. Pat. No. 2,904,511), that pure water from salt or brackish water is the target. The most conspicuous with this patent is that they use natural gas to form the hydrate, whereby two goals are achieved; pure water and pure natural gas. Natural gas will from the gas reservoir have a complex make-up with a content of more heavy hydrocarbons. Before transportation of LNG it will be advantageous if the more heavy components were gone, and this can be achieved through hydrate formation. Although they mention that contaminated water can pass through the process several times; the fact that the increased amount of salt will lead to a stop in the process is not taken into account. Nor is there paid any interest in what happens with the contaminated water leaving the process.
A further reference describing earlier technique is of a more recent date, from 2001: PCT/US01/27720, with the heading: “Improved hydrate desalting for water purification”. Focus in the reference is the temperature control in the water phase which is going to be purified to get an efficient process with regard to energy balance. The process is focusing only on the production of pure water for consumption without paying any attention to what has to be done with the contaminants.
Norwegian patent NO 311.854 concerns a method and a system for transportation of a stream of fluid hydrocarbons containing water. In this patent focus is directed towards the continuous formation of gas hydrates in a process stream to ensure good transportation of fluids containing water in pipelines through areas with low temperature. The hydrates are harvested out of the transportation pipeline in such a way that no hindrances arise in the pipeline transport and in such a manner that continued hydrate formation from the sources are secured.
Common for earlier technique is that it is not focused on what shall happen with contaminants and salts which are removed from the water. Further, none of the references from earlier technique are directed towards the purification of the production water from the oil industry. The water they are directing their attention to is sea water, brackish water or contaminated surface water. Thereby they are not paying any attention to the fact that industrial processes can get into a situation where they have to make a halt because they contaminate the nature.
Nothing has been found in earlier technique concerning the handling of the contaminants. On the other hand, it is at least of equal importance that nothing in earlier technique regarding water purification concerns the uncertainty around the nucleation of hydrates. In this area, the present invention presents a significant improvement.
Within the oil production it is traditionally endeavoured to avoid hydrate formation because hydrates implies the risk of clogging of pipelines, which leads to operational problems and can constitute a security risk.
The object of the present invention is exactly that industrial processes should be made both ecological acceptable and cost efficient in the same operation. The primary object of the present invention is linked to production of oil/gas, but even if the invention is described mainly with reference to oil/gas production, it is equally relevant for other types of industries—for instance paper production, recovery plants for nuclear waste, spills from ships to sea, recirculation of water during space travel, etc.
It is therefore a need for a method for absolute purification of contaminated water. The purpose is to produce completely pure water which can be discharged into the nature without any risk for the environment. The process will not always be able to purify the whole amount of water, there will be a rest which will be strongly concentrated with contaminants. Because the concentration increases, other methods can be used with considerably higher efficiency and thereby contribute to a cost effective production of e.g. oil/gas, even if the amount of water in the production stream increases further. The final rest of contaminated water phase can e.g. be reinjected in the reservoir without any risk for the nature. It is obvious that this process can be utilized to satisfy many industrial and social needs.
An important aspect in addition to the requirement of making the process effective and thereby less energy consuming, is the question of how to utilize all the waste heat. The waste heat could be used as remote heating both in housing areas on oil platforms and in other installations where heat supply is required. Cables could in principle be built down to wellheads as well as pipelines for the heating of these so that plugging is avoided. In any case, some means could reduce those costs, both by the methods and devices according to the present invention and generally in the constructions on the platforms.